MAGNETICALLY COUPLED
CIRCUITS
BIOT-SAVART’S LAW
The magnetic field intensity dH produced at a certain point P, by
the differential current element Idl is proportional to the product
Idl and the sine of the angle α between the element and the line
joining P to the element and is inversely proportional to the square
of the distance R between P and the element.
dH ∝
dH =
I dl sin α
R2
I dl sin α
4π R 2
The direction of dH is determined by the right hand rule (RHR)
v
v v
v
ˆ
v I dl × R I dl × R
v
I dl × Rˆ
dH =
=
→ H=∫
(line current)
2
3
2
4π R
4π R
L 4π R
FARADAY’S LAW
• The physical or experimental law governing the principle of
magnetic induction .
• “The electromotive force (EMF) induced in a circuit is directly
proportional to the time rate of change of magnetic flux through
the circuit.”
• The EMF can either be produced by
changing B (induced EMF) or by
changing the area, e.g., by moving the
wire (motional EMF).
SELF INDUCTANCE
• According to Faraday’s law, the voltage induced in a coil is
proportional to the number of turns N and the time rate of
change of the magnetic flux φ.
dφ
dφ di
di
v=N
=N
=L
dt
di dt
dt
• L is the inductance of the inductor commonly called selfinductance (relating the induced voltage in a coil by a timevarying current in the same coil)
MUTUAL INDUCTANCE
• Two coils in a close proximity are linked together by the
magnetic flux produced by current in one coil, therby inducing
voltage in the other.
→ the two coils are said to be magnetically coupled although
they are physically apart.
• MUTUAL INDUCTANCE is the ability of one inductor to
induce a voltage across a neighbouring inductor, measured in
henrys (H).
• Mutual coupling only exists when the coils are in close
proximity, and the circuits are driven by time-varying sources.
di1
dφ1 di1
v1 = N1
=L
di1 dt
dt
dφ12 di1
di1
= M 21
v2 = N 2
di1 dt
dt
M21 is mutual inductance of
coil 2 with respect to coil 1
di2
dφ2 di2
=L
v2 = N 2
di2 dt
dt
di1
dφ21 di2
= M12
v1 = N1
di2 dt
dt
M12 is mutual inductance of
coil 1 with respect to coil 2
DOT CONVENTION
• M12 = M 21 = M and is always a positive quantity.
di
• The induced voltage M
may be positive or negative.
dt
• The choice of polarity is made by examining the way in which
both coils are physically wound and applying Lenz’s law in
conjunction with the right-hand-rule.
• The procedure is inconvenient in circuit analysis since it is
difficult to show the construction details of the coil in circuit
schematics. → use the dot convention (often predetermined)
• If a current enters (leaves) the dotted terminal of one coil, the
reference polarity of the mutual voltage in the second coil is
positive (negative) at the dotted terminal of the second coil.
TIME-DOMAIN ANALYSIS
di1
di
+M 2
dt
dt
Applying KVL to coils 1,
v1 = i1R1 + L1
Applying KVL to coils 2,
di2
di1
v2 = i2 R2 + L2
+M
dt
dt
FREQUENCY-DOMAIN ANALYSIS
V 1 = ( R1 + jω L1 ) I 1 + jω M I 2
V 2 = jω M I1 + ( R2 + jω L2 ) I 1
Ex. Practice Problem 13.1
Determine the voltage V o in the circuit shown.
Ex. Practice Problem 13.2
Determine the phasor currents I 1 and I 2 in the circuit shown.